ABSTRACT

ASTM A9231 provides three test methods to detect the presence of intermetallic precipitation in Duplex and Super Duplex Stainless Steels (DSS & SDSS). While the ASTM standard was initially developed for mill products (base metals), it has become the standard used to qualify Weld Procedure Specifications for DSS and SDSS fabrication. While Test Method A (electrolytic NaOH) etch can be used as a screening test, it is sometimes not permitted for this purpose by User specifications. This leaves Test Method B (Charpy “V” notch (CVN) testing) and Test Method C (ferric chloride exposure/corrosion rate testing) to qualify the welding procedure for appropriate metallurgical structure. For 25% Cr SDSS welds, Test Method C is difficult to pass at 40°C, and is essentially non repeatable. While it is an effective test in determining the extent of secondary phase particle precipitation (intermetallics and nitrides) in base metals, it is not appropriate for 25% Cr SDSS weldments, especially the root pass. Data presented in this paper will illustrate the problems using ASTM A923 Test Method C for weld procedure qualification for 25% Cr SDSS.

SUMMARY

Use of DSS has increased during the last 25 years, much of that growth due to its high strength and corrosion resistant properties for sub-sea application. Recently, use of these alloys has met with challenges, many of them having to do with welding operations.

The use of DSS, and specifically that of Super Duplex Stainless Steels while expanding, has not been without its growing pains. As components in thicker sections were developed for expanding markets, the requirements necessary to obtain a good weld were similarly growing. The most recent edition of API 5822 has expanded to include additional process requirements for welding DSS and SDSS (6.5), additional requirements for Weld Procedure Qualification (11.3), and additional requirements for DSS and SDSS Production welds (12.8). Amongst these is the requirement for corrosion testing during weld procedure qualification per ASTM A923, Test Method C.

ASTM A923 (1998) was implemented to assess the presence of intermetallic phases in base material, but has also been used for weld qualification. It is well known that the precipitation of intermetallics such as sigma phase and chi phase can promote loss of toughness, and loss of corrosion resistance. ASTM A923 includes three separate test methods that can be used to assess the extent of intermetallic precipitation, namely:

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